Composite aluminum material



y 9, 1964 H. I.. came, JR 3,133,796

COMPOSITE ALUMINUM MATERIAL Filed July 19, 1961 l0 8 2) 6 f/% A INVENTOR HOWARD LEE CRAIG, JR.

that the glass surface is not intact.

United States Patent 3,133,796 (IUMPOSITE ALUMINUM MATERIAL Howard Lee Craig, J12, Henrico County, Va, assignor to Reynolds Metals Company, Richmond, Va, a corporation of Delaware Filed July 19, 1961, Ser. No. 127,441 3 Claims. (Cl. 29-1975) This invention relates to aluminum-base alloys and to composite aluminum-base metals having improved resistance to corrosion. These materials are especially suited for uses involving exposure to supply waters, as for domestic and industrial Water heaters and associated water handling equipment.

Home hot water heater tanks of cast-iron or galvanized steel construction are generally unsatisfactory because of susceptibility to corrosion which necessitates frequent replacement. The product of corrosion contaminates the water and is the source of another main objection to use of tanks of this type construction. Tanks of galvanized steel construction are limited to a maximum water temperature of about 140 F furthermore, since at higher temperatures the zinc no longer protects the steel body of the tank and rapid corrosion ensues. This temperature limitation is a considerable disadvantage because hotter water is often required, as for eflicient operation of home dishwashing equipment. These problems can be avoided through use of premium quality home hot water heater tanks which are commercially available and which are frequently installed as replacements for galvanized or cast iron home hot water heater tanks after their failure.

One type of premium tank which is sold with a guaranteed service life of fifteen years is constructed from glass-lined steel. Tanks of glass-lined steel construction, although generally satisfactory, are more expensive than the galvanized steel tanks, are heavier and consequently more diflicult to handle without specialized equipment during installation, and normally require the use of sacrificial anodes. In addition, it is extremely difiicult to manufacture a closed vessel having a glass lining which is free of flaws and imperfections. A glass lining is fragile and is susceptible to chipping, cracking and spalling, due to normal knocks and bumps incidental to shipment, storage, and installation. Once the integrity of the glass lining is destroyed, through any cause, corrosion of the steel shell follows rapidly and proceeds to the point where the shell is pierced. The tank must be replaced when the shell is pierced as repair in the field is generally not practical. To assure an adequate service life, sacrificial anodes may be employed which corrode preferentially and protect the metallic shell in the event When anodes such as magnesium are used, the hydrogen which forms in the tank during the corrosion of the magnesium presents a potential explosion hazard unless proper venting is provided. An additional disadvantage of the glass-lined steel construction is poor heat transfer characteristics. When using a direct fired burner for heating a glass lined hot water heater tank, it is necessary to operate the burner at a higher temperature than for a galvanized .steel tank in order to obtain the same internal tank surface temperature. Because the tank internal surface temperature determines the rate of heating of the contents and, ultimately, the maximum temperature of the contents, it is obvious that the glass-lined steel tank is at an additional disadvantage in this respect. More fuel must be consumed, furthermore, since a hotter flame means more heat lost up the flue in heating the glass-lined steel tank at Copper construction tanks and Monel metal construction tanks have both been offered, but the prohibitively high cost of these metals precludes their extensive use.

Aluminum tanks possess the advantages of lightness of weight, which reduces the labor and cost of installation, heat transfer properties which are even superior to those of galvanized steel and cast-iron, and generally good resistance to corrosion. Moreover, any corrosion products which do form are colorless and non-toxic. Aluminum hot water heater tanks operate satisfactorily, furthermore, at water temperatures of F. Home hot water heater tanks of aluminum are conventionally fabricated from clad alloys composed of two or more tenaciously adhering layers each contatining aluminum as the major constituent.

A typical composite for use in hot water heater tanks has a high strength aluminum alloy core for structural strength and an aluminum-plus-zinc alloy cladding to protect the core from corrosion. One such clad composite has a 6061 aluminum alloy core and a 7072 aluminum alloy cladding. 6061 aluminum alloy is the Aluminum Association designation for a heat-treatable wrought aluminum-magnesium-silicon alloy having a nominal composition of 1.0% magnesium, 0.6% silicon, 0.25% copper and 0.25%chromium; and 7072 aluminum alloy has a nominal 1% zinc in a commercial purity aluminum base. However, home hot water heater tanks fabricated from this cladding-core combination do not give satisfactory service in many sections of the country.

It is a particular object of this invention, therefore, to provide novel aluminum alloys and aluminum-base metals having improved resistance to corrosion in supply waters. A further object is to provide a novel composite aluminum product in which solution potential between the core composition and the cladding is sufiicient to provide sacrificial protection of the core over a broad pH range.

It has been found that greatly improved corrosion re sistance is exhibited by cladding-core combinations in which the cladding contains about 1% zinc and no more than 0.01% copper. Such a composition is conveniently obtained by alloying about 0.8-1.3% zinc with aluminum of at least 99.8% purity. In addition, the result is further enhanced by employing a core alloy of aluminum which contains from about 0.6% to about 1.2% copper. Such a system makes possible the attainment of sufficient potential difference between the cladding and the core that satisfactory protective current is developed over a wide pH range. This assures that the cladding will remain anodic to the core when the composite is exposed to any of a variety of supply waters encountered in the use of domestic and industrial water handling equipment.

A simple utilization of the invention may be illustrated by appropriate modification of the standard 6061-7072 NOTE.Composition is percent Maximum, unless shown as a range.

The potential difference between such cladding and core compositions is in the order of 0.15 to 0.20 volt, compared to typically less than 0.10 volt for the standard 6061-7072 coupling. (These values are based upon the PROTECTIVE CURRENT DEVELOPED (MICRO- AMPS) Core Percent Copper Conven tional 0.6 0.8 1.13

(a) Conventional (7072) 330 140 470 550 Special 0.01% Cu) 140 600 700 These results are quite conservative since the 7072 alloy employed actually analyzed at 0.04% copper, well within the published limit of 0.10% maximum.

The criticality of copper relative to other impurities introduced into the cladding by way of the aluminum is further emphasized by other findings. For example, it has been found that the probability of pitting (in a cladding composition composed of aluminum and a nominal 1% zinc) is reduced to a level corresponding to maximums of 0.10% each of silicon and iron only by limiting the copper content to below 0.01%, whereas a copper content in the range 0.01-0.l0% increases the pitting probability to a level about ten times as great as the probability corresponding to similar amounts of iron or silicon.

The concept of limiting impurities in the cladding (especially the copper content), and particularly together with employing a carefully controlled copper addition to the core, has been found to be applicable to a wide range of clad products. In general, the core alloy may be based upon the wrought aluminum alloys of the SXXX, SXXX, 6XXX and 7XXX series (having a nominal composition which includes at least one element selected from the group consisting of magnesium, silicon, zinc and manganese), as Well as the aluminum-silicon casting alloys. Thus, the core may be composed of an aluminum alloy having its major alloying constituent selected from the group consisting of magnesium, silicon, zinc and manganese, provided the electrode potential thereof is in the range of about -0.8 to -0.9 volt. Included are: (a) commercial alloys such as 3003, 3004, 5050, 5155, 6061, 7075 and 7178; (b) the wrought aluminum-magnesiumsilicon alloys generally (including 6053, 6062, and 6063); and (0) various casting alloys such as 214, 355, 356 and 360. Where such an alloy normally contains less copper than about 0.6% it is advantageous in the practice of the invention to modify the base alloy by addition of copper to achieve the prefer-red range, even if the resulting potential is somewhat more cathodic than 0.8 volt.

When the development of optimum physical properties is a consideration, a heat-treatable alloy is ordinarily required for the core. (The term heat-treatable is used in the accepted sense for alloys which are hardenable by thermal treatment.) For that purpose, the preferred wrought alloys are those of the aluminum-magnesium-silicon and the aluminum-zinc type (i.e., the 6XXX and 7XXX series alloys). Of the casting alloys, the most practical choices are the aluminum-silicon-magnesium type (such as 355 and 356). Consequently, the preferred heat-treatable core alloys contain up to about 1.5% magnesium (e.g., 0.40-1.2%) and have a silicon content up to about 1.8% (e.g., 0.45-1.4%) for wrought alloys and in the range of about 5-12% (e.g., 59.5%) for casting alloys. Other alloying elements such as chromium and manganese (ordinarily in amounts less than 1%) may also be present, particularly in the wrought alloys.

Accordingly, the invention contemplates improved clad composites wherein the core alloy is one of numerous aluminum alloys which are readily clad with aluminum alloyed with about 1% zinc, the beneficial result stemming largely from control of copper in the cladding. Particularly in the case of wrought heat-treatable aluminummagnesium-silicon alloys, furthermore, additional improvement results from controlled additions of copper to the core alloy.

For a better understanding of the invention and its various objects, advantages and details, present preferred embodiments thereof will be described with reference to the accompanying drawing.

In the drawing:

FIG. 1 is a fragmentary semi-schematic view in cross section of a clad aluminum composite in accordance with the invention, illustrating protection of the core by the cladding.

FIG. 2 is a similar view illustrating penetration of a core layer by corrosion pitting.

Referring to FIG. 1, cladding layer 2 is shown bonded to core layer 4 along the line 6 representing the face of said core layer. A corrosion pit 8 is shown penetrating the cladding layer to exposed surface 10 of the core layer. By reason of preferential corrosion of the cladding layer, the cathodic core layer is accordingly protected. Thus, the exposed face of core layer 10 (at the bottom of the corrosion pit which is on the plane of bonding depicted by line AA) is not penetrated.

In FIG. 2, there is represented the undesirable situation in which the cladding 2 is cathodic and the core 4 is anodic, with preferential corrosion of the core. As a result, corrosion of the core layer occurs, with an irregular crater 12 being formed at the bottom of the corrosion pit 8. When 7072 aluminum alloy cladding is used in combination with a 6061 aluminum alloy core material, corrosion usually takes the form illustrated in FIGURE 2. In accordance with the invention, however, the pitting normally takes the form illustrated in FIGURE 1, even in supply waters that cause the form of pitting illustrated in FIGURE 2 to occur in the 7072- 6061 cladding-core composite of the prior art.

The following examples serve to illustrate the present invention:

Example 1 A series of tests were performed, using city water which was alkaline and included about an equal proportion of carbonate and noncarbonate hardness. After one year of exposure, during which water temperatures averaged about E, eight sections of two different water tanks were submitted to metallographic examination to determine the depth of pitting in each of the sections. Four samples were from a tank made from commercial 7072 alloy (containing 0.04% copper) clad on commercial 6061 alloy (copper 0.15-0.40%), while four samples were from a tank made from modified 7072 alloy (less than 0.01% copper) clad on an alloy similar to 6061 but containing 0.8% copper. Pit depths ranged from 0.0129 inch to 0.0181 inch in the higher copper content core tanks, as compared with depths of 0.0440 inch to 0.0570 inch in the case of tanks made from standard alloy.

Example 2 Further tests were carried out to evaluate the longterm corrosion effects for certain core-cladding combinations, using supply waters of 22 cities in the United States. Each of these tests was performed in a tank in which the water was frequently changed. The tanks were located in the respective cities and were furnished with the local supply water. Triplicate specimens were exposed for a one-year period, and then examined for penetration of pitting into the core alloy below the cladding-core bond. All specimens having the composition indicated below showed the desired behavior (typified in FIGURE 1):

In the case of a conventional 6061-7072 combination, taken at random from commercial production, the results from 18 of the 22 cities showed pitting into the core (FIG. 2).

Example 3 The following table shows the effect of copper in the cladding alloy on the solution potential, the effect of copper in the core and the resultant solution potential difference. The solution potentials were measured in a one normal sodium chloride solution containing 0.3% hydrogen peroxide against a one-tenth normal calomel electrode:

Cladding Percent Sol. Core Percent Sol. Pot.

Cu Pot, v. Cu P0t., v. Difi.,v.

.003 -0. 09 0 6 0.S0 0.19 .005 0.90 0 8 -0. 79 0.17 .01 -0.97 1 13 -0. 78 O. 19 .03 0.92 0 3 -0.82 0.10 .05 0.S9 0 3 0.82 0.07

Alloys A-E contained, in addition to copper, 08-13% zinc, and no more than 0.10% iron, 0.10% silicon, 0.02% others (each), 0.05% others (total), balance aluminum. The solution potential differences in alloys pairs D:K and E:K is indicative of the result obtained with conventional 6061-7072 composites.

Alloys G, H, and J contained, in addition to copper, 08-12% magnesium, (MO-0.8% silicon, 0.15-0.35% chromium, and no more than 0.7% iron, 0.25% zinc, 0.15 manganese, 0.15% titanium, 0.05% others (each), 0.15 others (total), balance aluminum.

While present preferred embodiments and practices of the invention have been described, it will be understood that the invention is not limited thereto but may be variously embodied and practiced within the scope of the following claims.

What is claimed is:

l. A composite aluminum-base metal exhibiting superior corrosion resistance, comprising an aluminum alloy core and a cladding on at least one face of the core, the core being composed of an aluminum-magnesium-silicon alloy consisting essentially of about 06-12% copper, up to about 1.5% magnesium and up to about 1.8% silicon, balance substantially aluminum, said core alloy being further characterized by having a solution potential of about -0.8 volt, and the cladding consisting essentially of aluminum alloyed with about 1% zinc, the diiference in solution potential between the cladding and the core being sufficient that the cladding remains anodic to the core during prolonged exposure of the composite to supply water, said cladding thereby providing sacrificial protection of the core.

2. A composite aluminum-base metal exhibiting superior corrosion resistance, comprising an aluminum alloy core and a cladding on at least one face of the core, the core being composed of an aluminum-magnesium-silicon alloy consisting essentially of about 0.6-1.2% copper, 08-12% magnesium, 0.40-0.8% silicon and 015-035 chromium, balance substantially aluminum, said core alloy being further characterized by having a solution potential of about -0.8 volt, and the cladding consisting essentially of aluminum alloy with about 1% zinc, the diiference in solution potential between the cladding and the core being sufficient that the cladding remains anodic to the core during prolonged exposure of the composite to supply water, said cladding thereby providing sacrificial protection of the core.

3. A composite aluminum-base metal exhibiting superior corrosion resistance, comprising an aluminum alloy core and a cladding on at least one face of the core, said cladding consisting essentially of an aluminum-Zinc alloy having about 08-13% zinc and no more than 0.01% copper, the balance being aluminum of at least 99.8% purity, the core being composed of an alloy which is predominantly aluminum, and has a solution potential of about 0.8 volt, said alloy consisting essentially of about 0.6-1.2% copper and an intentional addition of at least one alloying constituent selected from the group consisting of magnesium, silicon, zinc and manganese, balance substantially aluminum, the difference in solution potential between the cladding and core being sufficient that the cladding remains anodic to the core during prolonged exposure of the composite to supply water, said cladding thereby providing sacrificial protection of the core.

References (Zited in the file of this patent UNITED STATES PATENTS 2,100,257 Larson Nov. 23, 1937 2,208,186 Igarashi July 16, 1940 2,376,681 Gauthier May 22, 1945 2,742,688 Nock Apr. 24, 1956 

1. A COMPOSITE ALUMINUM-BASE METAL EXHIBITING SUPERIOR CORROSION RESISTANCE, COMPRISING AN ALUMINUM ALLOY CORE AND A CLADDING ON AT LEAST ONE FACE OF THE CORE, THE CORE BEING COMPOSED OF AN ALUMINUM-MAGNESIUM-SILICON ALLOY CONSISTING ESSENTIALLY OF ABOUT 0.6-1.2% COPPER, UP TO ABOUT 1.5% MAGNESIUM AND UP TO ABOUT 1.8% SILICON, BALANCE SUBSTANTIALLY ALUMINUM, SAID CORE ALLOY BEING FURTHER CHARACTERIZED BY HAVING A SOLUTION POTENTIAL OF ABOUT -0.8 VOLT, AND THE CLADDING CONSISTING ESSENTIALLY OF ALUMINUM ALLOYED WITH ABOUT 1% ZINC, THE DIFFERENCE IN SOLUTION POTENTIAL BETWEEN THE CLADDING AND THE CORE BEING SUFFICIENT THAT THE CLADDING REMAINS ANODIC TO THE CORE DURING KPROLONGED EXPOSURE OF THE COMPOSITE TO SUPPLY WATER, SAID CLADDING THEREBY PROVIDING SACRIFICIAL PROTECTION OF THE CORE. 